To obtain high quality images from cameras, it is desirable to collect as much light from a scene as possible. It is desirable to channel the light impinging the surface of an image sensor to an active pixel element for maximum detection and signal strength. Known image sensors do not have a fill factor (FF) of close to 100% in their microlens arrays. The FF is defined as the ratio of the amount of area from which light is collected and detected to the area of the image sensor.
There are two known methods used to form light concentrating elements called microlenses. The most common method is to expose and develop cylindrical patterns of photoresist, using one cylinder per unit cell. A photoresist reflow is performed to change the cylinder to a hemisphere. The reflow uses a time/temperature combination to melt the photoresist. A typical condition used is 60 seconds at 170° C. The fill factor using this technique is limited to approximately 78%, because the footprint of the hemispherical shape is circular, and, because a square array of microlenses is desired, thus, there are unused areas at the unit cell corners. This is a problem of fitting circles in squares. This is a cost effective method of forming a microlens, yet it lacks the ability to collect all the available light.
The other method is using a grayscale mask directly to pattern the photoresist, without using a reflow process. In this method, the photomask used is a special grayscale photomask. U.S. Pat. No. 6,562,523 B1, granted May 13, 2003, to Wu et al. for Direct write all-glass photomask blanks, and U.S. Pat. No. 6,524,756 B1, granted Feb. 25, 2003, to Wu for Grayscale all-glass photomasks describe these processes. The photomask described in the patents is essentially planar. A layer of about 1 μm in thickness, of zinc silicate, is exposed by an electron beam, which converts the transmission property, making the layer more opaque, depending on the electron beam dose. By making each of the microlenses of varying transmittances the resultant shape after expose and development is a true 100% fill factor of the desired shape. The problem with this method is the high cost of the photomask, which is too expensive for practical commercial use. This is because the process of making the photomask is a serial one, i e., exposing each microlens in succession to multiple doses of the electron beam.
U.S. Pat. No. 5,536,455, granted Jul. 16, 1996, to Aoyama et al. for Method of manufacturing lens array describes combination of plural lens-base elements.
U.S. Pat. No. 6,221,687 B1, granted Apr. 24, 2001, to Abramovich for Color image sensor with embedded microlens array describes use of a SixNy layer and reactive ion etching thereof to from a microlens array.
U.S. Pat. No. 6,473,238 B1, granted Oct. 29, 2002, to Daniell for Lens Arrays describes a lens array having air gaps therein.
U.S. Pat. No. 7,068,432 B2, granted Jun. 27, 2006, to Boettiger et al. for Controlling lens shape in microlens array describes adjusting melting properties of microlens material.
U.S. Pat. No. 7,078,260 B2, granted Jul. 18, 2006, to Jeon for CMOS image sensors and methods for fabricating the same describes a lens array wherein the lenses have a substantially flattened top surface.
U.S. patent application Ser. No. 11/588,891, filed Oct. 27, 2006, of Ono et al., for Grayscale Reticle for Precise Control of Photoresist Exposure, describes use of a low-cost technique for fabricating a grayscale mask for use in IC lithographic processes.